The present invention relates to a stepping motor drive circuit.
There are circuits for driving a stepping motor which is rotated and driven in response to an excitation phase switching signal or a step pulse signal supplied from an external control section. In such a drive circuit, generally, a switching device such as a transistor or the like which is turned on or off by the excitation phase switching signal is inserted in each excitation voltage supply path of each excitation coil constituting each excitation phase of the stepping motor, and when a level of the excitation phase switching signal is changed to switch the excitation phase, an excitation current flows through a corresponding excitation coil from a power source. Since the excitation coil has an inductance and an internal resistance, however, this excitation current value does not immediately become a specified value after the conduction of the switching transistor but increases in accordance with a characteristic curve which is determined by a time constant of the inductance and resistance value.
In general, to rotate the stepping motor at a high speed, it is necessary to improve the rise characteristic of a current flowing through each excitation coil when the excitation phase is changed over. Therefore, there has been proposed a stepping motor drive circuit in which such a rise characteristic of the current is improved and a constant current control circuit for controlling a current flowing through the excitation coil to be kept constant while a voltage is being applied to the excitation coil through the conductive switching transistor is interposed between the power source and each excitation coil.
For example, some of the constant current control circuits are constituted such that a large current flows at the start of conduction of the switching transistor using a current amplifying transistor and after an elapse of a constant period of time, the current is reduced and thereafter it is suppressed to a constant value.
However, the stepping motor drive circuit having such a constant current control circuit still has the following problems which must be solved. Namely, in the constant current control circuit, the current amplifying transistor is operated to suppress the current value a constant period of time after the current starts to rise, resulting in an increase in generation of heat due to the collector dissipation. Therefore, it is required to arrange devices and design a heat radiation mechanism in consideration of heat radiation due to the operation for a long time. On the other hand, a service life of a device could be reduced due to the heat generation as well.